Zhang Zhaochong, Yan Jiaxin and Li Bo helped with the field sampling. Rob Newton, Dougal Jerram and Jason Hilton are thanked for their field assistance We thank reviewers Scott Bryan and Dougal Jerram for their critical and constructive comments. This study is supported by grants from the National Natural Science Foundation of China (Grant nos. 40872002, 40921062), Natural Environment Research Council of UK (Grant NE/D011558/1), "111" Project of Chinese State Administration of Foreign Experts Affairs (Grant B08030) and Hong Kong Research Grant Council (grant no HKU700204).